Generally speaking, Paul, Silas, and Timothy were pleased with the Thessalonian church. In their estimation, the qualities of the believers there were "evidence that God's judgment is right" and they were "counted worthy of the kingdom of God" (v. 5 NIV). What was this "evidence"?
First of all, their faith was "growing" (v. 3a). Secondly, their love for one another was "increasing" (v. 3b NIV); and thirdly, their commitment was ongoing (especially in the context of persecution, they were "enduring" — v. 4 NIV). Growing faith, increasing love, ongoing commitment — these qualities were the "evidence" that the Thessalonian believers were "worthy of the kingdom of God".
How does faith grow? Simply by input and output. You digest the Word and live the Word — you take it in and live it out. In other words, you learn and do.
So how does love increase? You practise what you preach. As you ingest the word and live it, you develop a high view of your neighbour — you seek his highest good. And the more you choose to do what's right and best for your neighbour, the more your love increases. The love of God is "shed abroad" from you to others.
And how does commitment relate to all this dynamic motion, this growing and increasing? Commitment is the everyday, day-after-day "track" on which faith and love move. It's closely related to hope; because you have a hope, you have the perseverance and patience to see that hope realized. You have Heaven in view, and you follow through.